Thanks dimitriosfl - Yes I suggest properly update and finalize your game then promote. One mistake that I did was during my Admob campaigns I was updating my game, and the problem is: not everyone updates! So I'm stuck with half my audience with Facebook share option while the rest are still playing an older version.
Well, campaign costing is a tricky question but let me summarize, based on my experience:
You are charged only when the person clicks on your ad. (You will not be charged if it showed 10,000 times and no one clicks). You can also see the "conversions" (how many people actually Installed your game among those clicks), this updates normally after 24 hours though.
The cost per ad is mainly dependent on the country where your Ad will be shown (due to the currency differences). I suggest narrowing it down by having different campaigns with selected (targeted) countries only. That helps a lot.
For example, Cricket games are most famous in India (comparatively speaking), and based on that country, I had to pay only $0.01 per ad click. Whereas, if you promote your game in UK, USA, Australia etc then set your AD/click to around $0.5, because for these countries it will charge your $0.1 to $0.5 per ad click.
Hopefully this will help..